Thermally isolated via structure

ABSTRACT

This document discusses, among other things, a flexible circuit or other laminate comprising a first conductive layer and a second conductive layer disposed over the first conductive layer. An insulator is disposed between the first and second conductive layers. A conductive via extends through the insulator and electrically connects the first and second conductive layers. The laminate includes a channel in the insulator. In one option, the channel extends at least part way around the via. In another option, the channel extends at least part way between the first and second conductive layers. In another example, a method comprises providing a laminate including at least first and second conductive layers and an insulator disposed therebetween. A via is formed through the insulator. A channel is formed in the insulator at least part way around the via. The channel extends between the first and second conductive layers.

TECHNICAL FIELD

This document relates generally to circuit boards and in particular toconductive vias.

BACKGROUND

Cardiac rhythm management (CRM) devices such as pacers, cardioverters,defibrillators, cardiac resynchronization therapy (CRT) devices, as wellas combination devices typically include flexible printed circuitboards. A printed circuit board is a laminate of dielectric layerssandwiching layers of conductive circuits. Conductive vias extendthrough the dielectric layers to electrically couple the circuits ofdifferent layers. The dielectric layers often have higher coefficientsof expansion compared to the conductive materials used in the vias. Thevias expand less in the presence of heat (e.g., heat from manufacturingprocesses or circuit operation) or moisture relative to the insulationmaterial used in the dielectric layers. Moreover, the dielectricmaterials sometimes include fibers or the like. The dielectric layersexpand more in directions orthogonal to these fibers and often expand ina direction substantially parallel to the vias.

Expansion of the dielectric layers applies stress along the lessexpansion prone vias. The stress is typically focused at the juncturesbetween the vias and their contacts to the conductive layers on thecircuit boards. Sufficient stress due to expansion of the dielectricmaterial separates the vias from such conductive contacts. This canreduce circuit performance or even cause an electrical open circuit thatcan ruin the circuit board. Additionally, separation of the vias andcontacts in manufacturing requires discarding of the affected circuitboards. This increases manufacturing costs. The present inventors haverecognized an unmet need for reducing stress on conductive vias toavoid, for example, circuit board failure and to improve manufacturingquality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a portion of a circuit boardlaminate.

FIG. 2 is a sectional view illustrating a portion of a circuit boardlaminate including a channel around a conductive via.

FIG. 3 is a perspective view illustrating a portion of a circuit boardlaminate including a channel around a conductive via.

FIG. 4 is a perspective view illustrating a portion of a circuit boardlaminate including a conductive via and a non-linear trace.

FIG. 5 is a sectional view illustrating a portion of a circuit boardlaminate including a filler material disposed in the channel.

FIG. 6 is a sectional view illustrating a portion of a circuit boardlaminate including a sealant disposed over the circuit board.

FIG. 7 is a perspective view illustrating a portion of a circuit boardlaminate including a conductive via and an adjacent opening containing asecuring filler.

FIG. 8 is a block diagram illustrating a method for making a circuitboard laminate.

FIG. 9 is a block diagram illustrating a method for processing a circuitboard laminate to avoid loss of contact between the conductive via and apad.

FIG. 10 is a block diagram illustrating a method for cycling electricityin a circuit board laminate to avoid loss of contact between theconductive via and a pad.

DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and that structuralchanges may be made without departing from the scope of the presentinvention. Therefore, the following detailed description is not to betaken in a limiting sense, and the scope of the present invention isdefined by the appended claims and their equivalents.

FIG. 1 is a sectional view of a laminate 100, for instance a flexible orother circuit board. The laminate 100 includes insulating layers 102 andconductive layers 104. In the example shown in FIG. 1, the laminate 100includes conductive layers 104A–C. The insulating layers 102 interposethe conductive layers 104A–C. In one example, the insulating layers 102include a dielectric such as polyimide or the like. In another example,the conductive layers 104A–C include a conductive material, forinstance, copper or the like. The conductive layers 104A–C andinsulating layers 102 are coupled together with heat, adhesives or thelike. At least one conductive via 106 extends at least part way throughthe laminate 100 to electrically couple the conductive layers 104A,104B. The conductive via 106, in one example, includes a conductivematerial such as copper. In another example, the via 106 is coupled toconductive layer 104A with a trace 108. The trace 108 extends betweenthe conductive layer 104A and the via 106 to provide an electricalcontact between the via 106 and the conductive layer 104A. Theconductive layer 104B is coupled to the via 106 either directly or witha pad 110, in yet another example. The conductive pad 110 provides anelectrical contact that electrically couples the conductive layer 104Bwith the via 106.

The via 106 is disposed within a cavity 112. The cavity 112 is sized andshaped to extend between the conductive layers 104A, 104B and throughthe intervening insulating layers 102. Typically, the via 106 is platedbetween the conductive layers 104A, 104B to form one electricalconnection between the conductive layers 104A, 104B. In another example,the via 106 electrically connects more than two conductive layers, forinstance conductive layers 104A–C. The via 106 typically is platedaround the inner surface of the cavity 112 and is along one or more ofthe insulating layers 102.

Relative to the conductive layers 104A–C and the via 106, the insulatinglayers 102 expand more when the laminate 100 is exposed to processingconditions such as heat, moisture, chemical solutions or the like.Additionally, the insulating layers 102 may expand when the conductivelayers 104A–C receive electricity and thereby generate heat, forinstance, during fabrication, testing, pacing, defibrillation, or thelike. As described above, the insulating layers include, in one example,a dielectric (e.g., polyimide). Such a dielectric, typically has athermal coefficient of expansion of approximately 30 to 200 ppm/degreeC. The conductive material (e.g., copper) used in the via 106 and theconductive layers 104A–C typically has a lower thermal coefficient ofexpansion, for instance, approximately 17 ppm/degree C.

During fabrication of the laminate 100, for example, temperatures canreach approximately 225 to 240 degrees Celsius to fuse the conductivelayers 104A–C to the insulating layers 102. The laminate 100 may alsoexperience increased temperatures when electrical power is applied tothe conductive layers 104A–C (e.g. pacing, defibrillation, testing orthe like). The insulating layers 102 expand more than the conductivematerial of the via 106. Because of the differences in thermal expansionof the insulating layers 102 and the conductive material in the via 106expansion of the insulating layers 102 tends to pull the via 106 awayfrom the pad 110. The stresses at the point of coupling between the pad110 and the via 106, in one example, can tear the via 106 away fromcontact with the pad 110, thereby increasing the electrical resistanceof the via. In another example, the stresses partially pull the via 106out of contact with the pad 110. Typically, separation of the via 106from the pad 110 creates a gap 114. The gap 114, in one example,prevents electrical communication between the via 106 and the pad 110and therefore prevents electrical communication between the conductivelayers 104A, 104B. In another example, the gap 114 is small enough thatat least a portion of the via 106 and the pad 110 remain in contact. Thegap 114 can widen and sever the connection between the via 106 and thepad 110 with age, additional power cycling, or processing steps.

In another example, the insulating layers 102 include a compositedielectric material. The composite dielectric typically includes fibers116 extending in a plane substantially parallel to the conductive layers104A–C. Heat or chemical solutions cause the insulating layers 102 toexpand to a greater degree in a direction orthogonal to the plane of thefibers 116 and to a lesser degree in a direction parallel to the planeof the fibers 116. In one example, the insulating layers 102 expand in adirection substantially parallel to the via 106. This increases thestress on the juncture between the via 106 and the pad 110 and can pullthe via 106 out of contact with the pad 110.

The insulating layers 102 also expand when exposed to wet cleaningsolutions. The materials of the insulating layers 102 absorb moistureand swell. The conductive material of the via 106 absorbs less moistureand does not expand to the same degree as the insulating layers 102.Moisture induced stress at the juncture between the via 106 and the pad110 is on the same order as that of the thermal expansion. The via 106,in one example, is torn or partially separated from the pad 110 as shownby the gap 114.

FIG. 2 is a sectional view of a laminate 200, for instance a flexible orother circuit board constructed with similar components as the laminate100, described above. In this example, however, the laminate 200includes a channel 202 extending at least part way around the via 106.In one example, the channel 202 extends between the conductive layers104A, 104B. In another example, the channel 202 extends the entiredistance between the conductive layers 104A, 104B. The channel 202extends around the via 106 and between the conductive layers 104A, 104B,in an example, far enough to reduce mechanical stress on the electricalconnection between the via 106 and the pad 110 coupled to the conductivelayer 104B due to swelling of the insulator. The mechanical stress isreduced enough to preserve electrical connectivity of the first andsecond conductive layers 104A, 104B by the via 106 in spite of thedielectric swelling, in another example, as described below. The channel202 is typically formed in the laminate 200 by plasma etching, chemicaletching, laser machining, or the like. In one example, a small amount ofinsulating material 203 is left underneath a rim of the via 106. Thechannel 202 substantially isolates the via 106 from the insulatinglayers 102. This substantially prevents stress and resulting separationof the via 106 from the pad 110 caused by expansion of the insulatinglayers 102. As a result, the via 106 experiences little or no stressthat would pull it out of contact with the pad 110. The channel 202substantially separates the via 106 from the insulating layers 102 andallows the via 106 to remain in electrical contact with the conductivelayers 104A, 104B during expansion of the insulating layers 102.

FIG. 2 and FIG. 3 illustrate the trace 108 that electrically couples thevia 106 to the conductive layer 104A. In one example, as shown in FIG.2, the trace 108 bridges the channel 202. In another example, multipletraces 108 extend from the via 106 and provide electrical contactbetween the via 106 and at least one of the conductive layers such asconductive layer 104A and other additional conductive layers. In yetanother example, some residue insulating material 204, remains beneaththe trace 108 and extends toward the conductive layer 104B. Theinsulating material, optionally extends between the trace 108 and theconductive layer 104B. Expansion of the remaining insulating material204 does not provide sufficient stress to separate the via 106 from thepad 110 because the insulating material 204 does not extend far enougharound the via 106.

FIG. 4 is a perspective view of the via 106 surrounded by the channel202. A non-linear trace 400 extends between the via 106 and theconductive layer 104A. The non-linear trace 400 spans the channel 202and provides electrical communication between the conductive layers104A, 104B along with the via 106. Expansion of the insulating layers102, even with the channel 202 can still cause stress along the lineartrace 108 (FIGS. 1 and 4). The linear trace 108 is put in tension by theinsulating layers 102 during expansion. The insulating layers 102 movethe conductive layer 104A with respect to the juncture between the via106 and the trace 108. The linear trace 108 is pulled by both the via106 and conductive layer 104A. Enough stress may separate the lineartrace 108 from the via 106 or conductive layer 104A, thereby severingelectrical communication between the conductive layers 104A, 104B.

The non-linear trace 400 has sufficient flexibility to remain coupledbetween the conductive layer 104A and the via 106. The non-lineargeometry of the trace 400 allows the trace 400 to expand when in tensionwithout separating from the conductive layer 104A or the via 106. In oneexample, the trace 400 has a corrugated geometry. As the trace 400 ispulled between the via 106 and the conductive layer 104A thecorrugations of the trace 400 extend and the trace 400 remains coupledto the via 106 and the conductive layer 104A. In another example, thetrace 400 includes but is not limited a curved, spiraled, zig-zag,serpentine or other geometry.

FIG. 5 is a sectional view of the laminate 200 including the channel202. The channel 202 and the cavity 112, in one example, are filled witha filler 500. The filler 500, in another example, fills the entirety ofthe channel 202 and the cavity 112. The upper surface 502 of the filler500 is coplanar with the conductive layer 104A in yet another example.As shown in FIG. 5, the upper surface 502 of the filler 500 extendsabove the plane of the conductive layer 104A. The filler 500 provides acover for the channel 202 and the cavity 112 and substantially preventscontaminants and the like from lodging within the channel and/or cavity112. In one example, the filler 500 is a pliable material having a lowerYoung's modulus than that of the via 106, insulating layers 102 orconductive layers 104A–C. Examples of the filler 500 include, but arenot limited to, silicone, epoxy or the like. When disposed in thechannel 202, during expansion of the insulating layers 102 the filler500 includes at least a portion that moves with the insulating layers102 and the conductive layers 104A–C and another portion that remainssubstantially stationary with the via 106. As a result the filler 500,in one example, is sufficiently pliable to stretch and move withexpansion of the laminate 200 to remain coupled around the via 106 andcoupled to the insulating layers 102 and conductive layers 104A–C.

In another example, the filler 500 includes a more rigid material, forexample copper, silicon filled epoxy, acrylic or the like. The filler500 is coupled to the conductive layers 104A–C, insulating layers 102and the via 106, in one example. Where the filler 500 is conductive(e.g. a copper paste), sufficient clearance is provided between thefiller 500 and the via 106 and/or the conductive layers 104A–C toprevent shorting across the filler 500. In another example, the filler500 is sufficiently rigid to secure the coupling between at least one ofthe conductive layers 104A, 104B and the via 106. As a result the filler500 provides a rigid buffer that restrains expansion of the insulatinglayer 102 around the channel 202. Restraining the expansion of theinsulating layers 102 allows the via 106 to remain coupled to the pad110. The stresses caused by the insulating layers 102 expanding aresubstantially mitigated by the buffer created with the rigid filler 500.Additionally, the filler secures the trace 108 extending between the via106 and the conductive layer 104A, in one example. As shown in FIG. 5,at least a portion of the filler 500 is disposed above the trace 108.The filler 500 is sufficiently rigid to restrain expansion of theinsulating layers 102 that apply stress to the trace 108, in anotherexample. The filler 500 substantially prevents separation of the trace108 from contact with the via 106 and/or the conductive layer 104A, inyet another example.

FIG. 6 is a sectional view of the laminate 200 including a filler 500and a sealant 600. In the example described above with an upper surface502 of the filler 500 disposed over the plane of the conductive layer104A, the sealant 600 is applied over the conductive layer 104A and thefiller 500 and defines a planar surface for the laminate 200. In anotherexample, the sealant 600 defines a non-planar surface. The sealant 600substantially separates the other components of the laminate 200 fromexposure to environmental conditions such as moisture, contaminants orthe like. In one example, the sealant 600 substantially prevents ingressof moisture to the insulating layers 102 and expansion of the insulatinglayers 102 because of moisture absorption. The sealant 600 includes, butis not limited to epoxy and acrylic solder masks or the like. Thesealant material, in an example, is applied as a liquid and cured withheat to form the sealant 600.

FIG. 7 is a perspective view of the laminate 200 including the channel202 and an opening 700. In one example, the opening 700 is substantiallyadjacent to the via 106 and the channel 202. The opening extends atleast part way through the insulating layers 102. The opening 700, inanother example is filled with an anchoring filler 702. The anchoringfiller 702 couples with at least the insulating layers 102. Theanchoring filler 702, in yet another example is sufficiently rigid torestrain expansion of the insulating layers 102 substantially adjacentto the via 106. The anchoring filler 702, includes but is not limited tocopper, cements or the like. As a result, the anchoring filler 702 incombination with the channel 202 provides enhanced protection for thecoupling between the via 106 and the pad 110. The channel 202 isolatesthe via 106 from expansion of the insulating layers 102, and theanchoring filler 702 in the opening 700 restrains expansion of theinsulating layers 102 around the channel 202. In another example,multiple openings 700 are disposed around the channel 202. The openings700 are filled with the anchoring filler 702 to provide additionalrestraint of expansion of the insulating layers 102.

FIG. 8 is a block diagram illustrating a method 800 for making alaminate, such as the laminate 200 shown in representative FIG. 2. Inone example, the laminate 200 is a printed circuit board. At 802 alaminate 200 is provided including first and second conductive layers104A, 104B. An insulator, for instance, insulating layer 102 is disposedbetween the first and second conductive layers 104A, 104B. At 804, a via106 is formed through the insulator.

At 806, a channel 202 is formed in the insulator at least part wayaround the via 106. Forming the channel 202 includes, in one example,disposing a mask (e.g. a copper sheet or photo mask) over the laminate.The mask includes an opening that substantially corresponds to an outerperimeter of the channel 202. Forming the channel 202 further includes,in another example, removing at least a portion of the insulator underthe opening. The insulator under the opening is removed with, forinstance, a chemical etch, laser machining, plasma etching or the like.Forming the channel 202 leaves the first and second conductive layers104A, 104B substantially unchanged under the opening. When plasmaetching is used, in one example, the plasma etch does not affect theconductive layers 104A, 104B. The channel 202 extends between the firstand second conductive layers 104A, 104B. In one example, the channel 202extends completely between the first and second conductive layers 104A,104B.

In another example, the channel 202 is filled with a pliable filler 500(see FIG. 5), for instance silicone or epoxy. In yet another example, afiller 500 is disposed within the channel 202 to securely couple atleast one of the first and second conductive layers 104A, 104B to thevia 106. Optionally, a sealant 600 (see FIG. 6) is disposed on thelaminate.

In yet another example, the method 800 includes acts for testing thecontacts between the via 106 and at least one of the first and secondconductive layers 104A, 104B. The laminate 200 is exposed to a dye. Thelaminate 200 is then exposed to a vacuum. In one example, the vacuumremoves air within a crack, such as gap 114, between a pad 110 and thevia 106 (see FIG. 1). The vacuum draws the dye into the gap 114 viacapillary action. The laminate 200 is then dried, for example, at 100degrees Celsius for 30 minutes. The via 106 is extracted from thelaminate 200. In one example, a wire is secured to the via (e.g. bysoldering) and the wire is pulled to extract the via 106. At least thevia 106 and/or first and second conductive layers are examined for thepresence of dye. In one example, the dye is disposed on the via 106and/or conductive layers 104A, 104B corresponding to the gap 114.

FIG. 9 is a block diagram illustrating a method 900 for processing acircuit board to avoid loss of contact between a via 106 and at leastone of first and second conductive layers 104A, 104B. FIG. 2 provides arepresentative example of the via 106 and conductive layers 104A, 104B.At 902, a laminate 200 is provided including first and second conductivelayers 104A, 104B. An insulator, such as insulating layer 102 isdisposed therebetween. At 904, a via 106 is provided that extendsthrough the insulator at least between the first and second conductivelayers 104A, 104B. At 906, the laminate 200 is processed such that theinsulator swells. In one example, the laminate 200 is heated. In anotherexample, the laminate 200 is heated to above about 240 to 250 degreesCelsius. In yet another example, processing the laminate 200 includesexposing the laminate to at least one cleaning solution (e.g. anacetone, alcohol or the like). Processing the laminate 200 includes,optionally, absorbing moisture in at least the insulator. In anotherexample, processing the laminate 200 includes swelling an adhesivebetween the insulator and at least one of the first and secondconductive layers 104A, 104B. Processing the laminate 200 includes, instill another example, swelling the insulator in a substantiallyperpendicular direction to a plane defined by a surface of at least oneof the first and second conductive layers 104A, 104B. In another option,the insulator swells substantially perpendicular to a plane of fibers116 (FIG. 1) within the insulator. The via 106 is isolated from theswollen insulator by a channel 202 extending at least part way aroundthe via 106. A pliable filler 500 (see FIG. 5) is disposed within thechannel 202 in another example. Processing the laminate, includes atleast a portion of the pliable filler 500 moving with the insulator andat least another portion of the pliable filler 500 remainingsubstantially stationary with the via 106.

FIG. 10 is a block diagram illustrating a method 1000 for cycling powerin the laminate 200 such that a via 106 remains in contact withconductive layers 104A, 104B. Laminate 200, the via 106 and conductivelayers 104A, 104B are shown in representative FIG. 2. At 1002, thelaminate 200 is provided and includes at least first and secondconductive layers 104A, 104B and an insulator (e.g. insulating layer102) disposed therebetween. At 1004, a via 106 is provided through theinsulator between at least the first and second conductive layers. At1006, electricity is provided to at least one of the first and secondconductive layers 104A, 104B such that the insulator swells, forinstance, because of heat within the conductive layers 104A, 104B. Thevia 106 is isolated from the swollen insulator by a channel 202extending at least part way around the via 106. A pliable filler, forexample filler 500 (FIG. 5) is disposed within the channel 202. In oneexample, where electricity is provided to at least one of the first andsecond conductive layers 104A, 104B, at least a portion of the pliablefiller 500 moves with the insulator and at least another portion of thepliable filler 500 remains stationary with the via 106.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reading and understanding theabove description. It should be noted that embodiments discussed indifferent portions of the description or referred to in differentdrawings can be combined to form additional embodiments of the presentapplication. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A laminate comprising: a first conductive layer; a second conductivelayer disposed over the first conductive layer; an insulator disposedbetween the first conductive layer and the second conductive layer; aconductive via extending through the insulator and electricallyconnecting the first and second conductive layers, at least oneconductive trace electrically connects the via to at least one of thefirst and second conductive layers, and the at least one conductivetrace extends non-linearly between the via and at least one of the firstand second conductive layers; and a channel in the insulator, thechannel extending far enough around the via and far enough into theinsulator to reduce mechanical stress on the electrical connectionbetween the via and at least one of the first and second conductivelayers due to swelling of the insulator, the mechanical stress reducedenough to preserve electrical connectivity of the first and secondconductive layers by the via in spite of the swelling.
 2. The laminateof claim 1, wherein the laminate includes a flexible circuit board. 3.The laminate of claim 1, wherein the conductive via includes a platedhollow conductive via.
 4. The laminate of claim 1, wherein theconductive via is coupled along at least a portion of the insulator. 5.The laminate of claim 1, wherein the insulator includes a polymer. 6.The laminate of claim 1, the channel continuously extending at leastpart way around the via.
 7. An apparatus comprising: an electronicdevice, the electronic device including: a flexible circuit including: afirst conductive layer; a second conductive layer disposed over thefirst conductive layer; an insulator disposed between the firstconductive layer and the second conductive layer; a hollow conductivevia extending through the insulator and electrically connecting thefirst and second conductive layers, wherein the conductive via iscoupled along at least a portion of the insulator, and at least oneconductive trace electrically connects the via to at least one of thefirst and second conductive layers; and a channel in the insulator, thechannel extending at least part way around the via, wherein the channelextends at least part way between at least the first and secondconductive layers.
 8. The apparatus of claim 7, wherein the electronicdevice is an implantable medical device.
 9. The apparatus of claim 7,wherein the electronic device is an external device for communicatingwith an implantable medical device.
 10. The apparatus of claim 7,wherein the channel extends between the first and second conductivelayers.
 11. The laminate of claim 7, wherein the channel includes anetched channel.
 12. The laminate of claim 7, the channel continuouslyextending at least part way around the via.
 13. A laminate comprising: afirst conductive layer; a second conductive layer disposed over thefirst conductive layer; an insulator between the first conductive layerand the second conductive layer; a hollow conductive via extendingthrough the insulator and electrically connecting the first and secondconductive layers, wherein the conductive via is coupled along at leasta portion of the insulator, and at least one conductive traceelectrically connects the via to at least one of the first and secondconductive layers; and a channel in the insulator, the channel extendingat least part way around the via, wherein the channel extends at leastpart way between at least the first and second conductive layers. 14.The laminate of claim 1 wherein the at least one trace extendsnon-linearly between the via and at least one of the first and secondconductive layers.
 15. The laminate of claim 1 wherein the at least onetrace bridges the channel.
 16. The laminate of claim 1, wherein thechannel surrounds the via.
 17. The laminate of claim 1, wherein thechannel extends between the first and second conductive layers.
 18. Thelaminate of claim 1, wherein the laminate includes a flexible circuitboard.
 19. The laminate of claim 1, wherein the insulator includes apolymer.
 20. The laminate of claim 1, wherein the channel includes anetched channel.
 21. The laminate of claim 1, the channel continuouslyextending at least part way around the via.
 22. The laminate of claim 1,further comprising a pliable filler in the channel.
 23. The laminate ofclaim 22, wherein the filler includes silicone.
 24. The laminate ofclaim 1, further comprising a filler in the channel to help secure atleast one of the first and second conductive layers to the via.
 25. Thelaminate of claim 24, wherein the filler includes copper.
 26. Thelaminate of claim 1, further comprising an opening extending through atleast the first conductive layer and the insulator, wherein the openingis substantially adjacent to the channel.
 27. The laminate of claim 26,further comprising a filler disposed within the opening to help secureat least one of the first and second conductive layers to the via.